RESEARCH ARTICLE Multiple-input-multiple-output/ diversity antenna with dual band-notched characteristics for ultra-wideband applications Sachin Kumar 1 | Gwan H. Lee 1 | Dong H. Kim 1 | Wahab Mohyuddin 2 | Hyun C. Choi 1 | Kang W. Kim 1 1 School of Electronics Engineering, Kyungpook National University, Daegu, Republic of Korea 2 Research Institute for Microwave and Millimeter-Wave Studies, National University of Sciences and Technology, Islamabad, Pakistan Correspondence Kang Wook Kim, School of Electronics Engineering, Kyungpook National University, Daegu, Republic of Korea. Email: kang_kim@ee.knu.ac.kr Funding information BK21 Plus Project funded by the Ministry of Education, Korea, Grant/ Award Number: 21A20131600011; National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Grant/Award Number: NRF- 2019M1A7A1A02085630 Abstract Design and implementation of a four-port ultra-wideband (UWB) multiple-input-multiple-output (MIMO)/diversity antenna with dual band-notched characteristics are presented. The proposed diversity antenna is composed of four identical rhombic-shaped monopole radiators arranged orthogonally to achieve better interelement iso- lation and polarization diversity. At each port, the MIMO antenna shows an impedance bandwidth (S 11 ≤ –10 dB) of 10.5 GHz (2.8-13.3 GHz) and an interelement isola- tion larger than 18 dB. Furthermore, in order to eliminate 3.5 GHz (Wi-MAX) and 5.5 GHz (WLAN) bands from the UWB range, the antenna radiators are loaded with elliptical complementary split ring resonator structures. The MIMO performance metrics such as isolation, envelope correlation coefficient, and apparent diversity gain are calculated and presented. The MIMO/diversity antenna prototype is fabricated, and experimental results are verified with simulated results. KEYWORDS CSRR, diversity, MIMO, notch, UWB 1 | INTRODUCTION After the allocation of 3.1 to 10.6 GHz frequency range for the ultra-wideband (UWB) communication by Federal Com- munications Commission (FCC) in 2002, 1 UWB systems have received substantial attention to achieve high data rate wireless transmission. The UWB systems are often used for high-speed short-range applications in microwave imag- ing, cognitive radio, wearable devices, sensing networks, wireless personal area networks, and so forth. 2,3 The UWB antenna is a key component used for transmission and reception of electromagnetic signals. The planar monopole antennas, because of their small size, light weight, low profile, high radiation efficiency, and easy integration with other circuits, are preferred for UWB trans- ceiver systems. 4,5 Recently, several UWB monopole antennas with circular, square, triangular, hexagonal, and trapezoidal geometries have been designed by various researchers. 6 The main problem associated with UWB structures, however, is their small transmission distance, which is because of low transmission power specified by the FCC. 7 Furthermore, the radio communication environments suffer from the problem of multipath fading, which leads to deterioration of the received signal strength. To overcome these challenges, UWB systems with several diversity schemes such as time, frequency, spatial, or polarization have been reported. 8,9 The spatial diversity involves spatial replication of antenna elements, usually at the receiver side. In addition, the polarization diversity is also employed in which multiple copies of the signal are received through antennas with different polarization. While designing multiple-input-multiple-output (MIMO)/diversity antenna systems, the placement of multiple antenna elements within a small region is the most difficult task. 10,11 This phenome- non becomes more evident in portable terminals, where limited space is provided, thus deteriorating the overall per- formance of the MIMO systems. Adding more radiating elements within a MIMO antenna improves the link reliability of the system. 12 Over the past few years, considerable research efforts have been made to deal with the problems of mutual coupling. The interelement coupling level can be reduced by the deployment of decoupling networks, 13 neutralizing lines, 14 electromagnetic band gap (EBG) structures, 15 parasitic Received: 22 April 2019 DOI: 10.1002/mop.32012 Microw Opt Technol Lett. 2019;1–10. wileyonlinelibrary.com/journal/mop © 2019 Wiley Periodicals, Inc. 1